Choosing the Technique
On the MCAT, separation questions test whether you can match a mixture’s key property difference to the best separation technique. This section is the decision framework.
The Master Decision Table
| Mixture contains | Best technique | Exploits |
|---|---|---|
| Two liquids with different bp (>25°C) | Simple distillation | Boiling point |
| Two liquids with similar bp (<25°C) | Fractional distillation | Boiling point |
| A compound with high bp + decomposition risk | Vacuum distillation | Boiling point (at reduced pressure) |
| A solid organic compound + impurities | Recrystallization | Solubility vs. temperature |
| Organic acid + neutral compound | Acid-base extraction | pKa (ionizability) |
| Organic base + neutral compound | Acid-base extraction | pKa |
| Acid + base + neutral in one organic mixture | Sequential acid-base extraction | pKa |
| Polar + nonpolar in organic solvent | Liquid-liquid extraction | Polarity |
| Multiple organics to analyze qualitatively | TLC | Polarity on silica |
| Multiple organics to purify preparatively | Column chromatography | Polarity on silica |
| Volatile small organics (MW < 500) | GC | Volatility + stationary phase interactions |
| Non-volatile or thermally labile | HPLC | Polarity with liquid mobile phase |
| Biomolecules (proteins, DNA) | Gel electrophoresis | Size (+/- charge) |
| Proteins by pI | IEF | Isoelectric point |
| Proteins by two dimensions | 2D-PAGE | pI + size |
Common MCAT Scenarios
Scenario 1: “A mixture contains a volatile ester (MW 74) and a less volatile alcohol (MW 116). What is the best separation method?”
Answer: Simple distillation. Ester bp ~60°C, alcohol bp ~125°C. Distill the ester first.
Scenario 2: “You have a mixture of aspirin (aromatic carboxylic acid) and benzocaine (aromatic amine ester) in dichloromethane. How do you separate them?”
Answer: Acid-base extraction. Aspirin’s COOH is deprotonated by NaOH → aqueous layer. Benzocaine’s amine is protonated by HCl → different aqueous layer. Three-way separation recovers each.
Scenario 3: “A crude product of a reaction contains the desired alcohol product and several byproducts. How to purify?”
Answer: Column chromatography. Load onto silica; elute with a solvent gradient; collect fractions; identify by TLC and combine those with only the desired product.
Scenario 4: “A bacterial cell extract contains dozens of proteins. How to analyze?”
Answer: SDS-PAGE (1D) for size-based analysis. 2D-PAGE (2D) for comprehensive proteomic analysis. Combined with Western blot for specific detection.
Scenario 5: “A DNA restriction digest produced fragments of 500 bp, 1500 bp, and 5000 bp. How to size-separate?”
Answer: Agarose gel electrophoresis. Low % agarose (0.8%) resolves the three fragments by size; ethidium bromide staining visualizes them under UV.
Multi-Step Strategy
Many real-world purifications use MULTIPLE techniques in sequence. Example: purifying a natural product from plant tissue:
- Grind plant material in a polar solvent to extract water-soluble compounds.
- Liquid-liquid extraction (water + ether) to separate hydrophilic from hydrophobic.
- Column chromatography on the organic layer to separate the target from other organics.
- Recrystallization (if solid) to purify further.
- HPLC for analytical purity check.
Special Cases
- Stereochemistry: separating enantiomers requires chiral chromatography (special chiral stationary phase). Ordinary HPLC cannot resolve enantiomers.
- Trace analytes: need sensitive detection methods (LC-MS, MS, fluorescence). Not an ordinary distillation/extraction.
- Large industrial scale: crystallization and distillation dominate; chromatography is too expensive at ton scale.
What Techniques Share
- All separations exploit a measurable property difference.
- Multiple separation cycles improve purity (with diminishing returns).
- No technique is 100% efficient - each has a characteristic yield.
- Technique choice depends on: scale, cost, speed, purity required, sample properties.